An Eddy is a term used to describe the trail left by the mixing of a fluid and its reverse current as an object moves through the fluid. On Earth, it can be observed when a water current flows around a rock as seen on a beach or a coastal shore.
The Fifth Doctor, when using the Ionograph to detect the ion trail of a spaceship or spacepod, likened this trail to that of an eddy, and indeed dubbed their iono-trail a "space eddy". (AUDIO: Hexagora)